Actually Corbyn has said Assange should not face extradition. He's one of the few politicians to have stated this.
Corbyn has to be careful. If he promised that Assange should be allowed his freedom if elected, then the present government and the US authorities will use every trick in the book to make sure he's extradited before any general election. In other words, too definitive a position may be detrimental to Assange's chances.